Photonics is the science of molding the flow of light. A photonics revolution is currently taking place and it is anticipated that the use of photons to carry information will be as important as electrons are in microelectronics. To realize this fundamental change photonic materials need to be developed that enable for example the generation, guiding, detection, amplification, attenuation, modulation, switching, filtering, coupling, splitting, shifting, frequency doubling and tripling and storing processing operations of light [Made to Measure: New Materials for the 21st Century, P. Ball, Princeton U Press, 1997].
A new material (and devices derived therefrom) that is expected to prove useful in this regard is the photonic crystal (PC). The structure of a photonic crystal is based on for example a lattice filled with regular arrays of micron size air holes. When the refractive index contrast (RIC) between the voids and the material comprising the lattice is sufficiently large the photonic crystal has the property of diffracting light according to Bragg's law. This leads to the development of a photonic band gap (PBG) in the photon density of states (PDS) and the inability of light to propagate in the photonic crystal at wavelengths corresponding to the PBG.
Photonic band gap materials, as disclosed in S. John, Phys. Rev. Lett. 1987, 58, 2486, and E. Yablonovitch, Phys. Rev. Lett. 1987, 58, 2059, are a new class of dielectrics which carry the concept of molding the flow of light to its ultimate level, namely by facilitating the coherent localization of light, see S. John, Phys. Rev. Lett. 1984, 53, 2169, P. W. Anderson, Phil. Mag. 1985, B 52, 505, S. John, Physics Today 1991, 44, (5), 32, and D. Wiersma, D. Bartolini, A. Lagendijk and R. Righini, Nature 1997, 390, 671. The underlying theory of operation of a photonic bandgap material is founded upon the scattering and interference of light in a periodic dielectric at a wavelength close to the lattice spacing of the photonic crystal. The existence of a PBG causes selective filtering of certain wavelengths of light while other wavelengths are transmitted. This provides a mechanism for the control and inhibition of spontaneous emission of light from atoms and molecules forming the active region of the PBG materials, and offers a basis for low threshold micro-lasers and novel non-linear optical phenomena.
Therefore, it is the PBG that enables the photonic crystal to manage photons in ways like the electronic band gap allows semiconductors manipulate electrons. By engineering the lattice dimensions, refractive index contrast and structural defects of the PC it can be designed to function as either passive or active photonic devices in the near infrared around 1.5 microns. This is the wavelength range most suitable for applications in optical telecommunications. The PC devices are easily scalable to a broad spectrum of applications in the near-infrared, visible, and ultraviolet, including future local-area optical networks using visible or near-infrared light.
In order to obtain experimental evidence of PBG in photonic crystals much effort has recently been directed towards the synthesis and fabrication of materials with structures required to show the effects of a PBG. These effects include the localization of light and inhibition of spontaneous emission, however this research is still in its infancy. This is mainly because of the technical difficulty of producing well ordered three-dimensional structures having dielectric periodicity on the order of the wavelength of visible and near infrared light.
A significant obstacle to the realization of these photonic capabilities is the lack of a proven route for synthesis of high quality, very large-scale PBG materials and patterns thereof with significant electromagnetic gaps at micron and sub-micron wavelengths. The method of micro-fabrication must also allow the controlled incorporation of straight and bent line and point defects, for optical circuitry, during the synthetic process as well as the inclusion of appropriately configured optical waveguides and fibers for optical coupling of light into and out of the different photonic elements.
The current state of development of PCs is similar to the early days of semiconductor technology. There are interesting analogies between semiconductor chemistry and physics aimed at microelectronics and photonic materials chemistry and physics directed at microphotonics. The consensus is that progress with photonic crystals directed at photonics will rest on the ability to control and understand the influence of various types of intrinsic defects (e.g. vacancies, dislocations, and stacking faults) and disorder (e.g. size, shape and positional imperfections, polycrystallinity, cracks, inhomogeneities of refractive index, density, volume filling, and etching) in a photonic lattice on the PBG. Not surprisingly, technologies founded on photonic crystals are presently facing scientific challenges like those confronted with semiconductors in the 1940's and 1950's. The real potential of photonic crystals will be realized by minimizing the effects of intrinsic defects and randomness in the photonic lattice and by learning how to incorporate specific structural defects in the photonic lattice to achieve a particular function. Structural defects in photonic crystals like points, lines and bends, facilitate photon confinement and enable the development of low threshold micro-lasers and highly compact circuits of light for producing and guiding light between photonic crystals devices, such as optical switches, variable attenuators, wavelength dispersion compensators and wavelength division multiplexers and demultiplexers. These developments could lead to a photonics technology based on highly compact optical circuits in which integrated photonic crystal devices are coupled to optical waveguides and fibers in alloptical computer and telecommunication systems.
Presently at least six main methods are providing the first examples of photonic lattices in the form of crystals and films for optical studies. These methods comprise micromachining, holographic photolithography, electrochemistry, field ion beam, glancing angle deposition and colloidal crystal self-assembly. Each approach has strengths and weaknesses, which are in the process of being defined and evaluated. It is not clear whether one or a combination of these methods of physically and chemically shaping materials into PC will dominate in this emerging field. For example, 3-D Lincoln Log (Wood-Pile) PC architectures, with structural defects have been micro-machined from high refractive index semiconductors like, silicon, gallium arsenide and indium phosphide. These structures have an omnidirectional PBG at near infrared wavelengths. Spiral PC structures made from silica have been made by deposition of appropriate gas phase precursors at a specified angle onto a lithographically patterned rotating substrate. Also, 2-D periodic arrays of air cylinders with structural defects have been made by electron beam lithography as well as photolithography and etching methods. Another method involves making a 2-D or 3-D array of vertically aligned air cylinders with straight or periodically modulated walls with structural defects by lithographic and photoassisted electrochemical or current modulated electrochemical patterning in silicon wafers. Field ion beam methods have also been utilized to drill microchannels at specified angles into a 2-D periodic array of vertically aligned air cylinders with structural defects made by electron beam lithography or electrochemical patterning. These structures display a pseudo-PBG in the near infrared spectral region. Holey glass fibres are flexible 2-D photonic crystals that guide light over long distances. Here, hand assembled glass rods are heated and pulled into fine strands of flexible glass fibers. 1-D photonic crystals are also of interest. Fiber Bragg gratings are commercially important and may be considered a 1-D photonic crystal. The gratings are formed by laser-imprinting of small periodic refractive index changes into the guiding core of glass fibers and serve to reflect a narrow spectrum of guided light. Multi-layered dielectric mirrors also behave as a 1-D photonic crystals and are based on moderate to large contrasts in refractive index between alternating thin films of optical materials.
Physical methods of shaping bulk semiconductors into photonic crystals involve complex, time consuming and expensive multi-step processing. By contrast, self-assembly of micrometer scale silica spheres into face centered cubic (fcc) colloidal crystals provides a simple, fast, reproducible and cheap materials chemistry approach to producing photonic crystals. To self-assemble a PC with an omnidirectional PBG, a fcc silica colloidal crystal may be used as a template to replicate the interstitial void space in the form of a high refractive index material like silicon. Extraction of the template leaves behind an inverted fcc colloidal crystal made of silicon. The silicon PC is impenetrable to near infrared light around 1.5 microns, the wavelength of choice for optical fiber communications.
While bulk photonic crystals will be useful in producing optical circuit components, the most promising photonic circuit elements will be composites comprising a photonic crystal integrated with a substrate such as an insulator semiconductor and coupled to optical waveguides or fibers. Therefore it would be very advantageous to find relatively simple, rapid, reproducible and inexpensive methods of producing photonic crystal films and patterns having controlled thickness, area, lattice structure and orientation either on the surface and/or embedded within insulating or semiconducting wafers to be used directly and/or as templates for the manufacture of integrated photonic crystal devices on or within a photonic chip. If these photonic crystals on or embedded within the substrates could be coupled with light sources, for example by being combined with optical waveguides or fibers for coupling light into and out of the on wafer photonic crystals, then such combinations would provide the basis for highly compact photonic circuit devices. Producing photonic devices from silicon-based photonic crystals would be a very significant commercial advantage since methods of fabricating such materials could be readily retrofitted into existing silicon chip fabrication facilities.
A convenient method of synthesizing such composites involves colloidal crystal self-assembly. A number of methods have been described in the literature for patterning micro-spheres made of silica and latex on the surface of various substrates. One method called colloidal epitaxy (A. van Blaaderen, R. Ruel and P. Wiltzius, Nature 1997, 385, 32) utilizes a photolithographically patterned array of micron scale holes in a polymer resist deposited on a planar substrate as a template to capture and organize micro-spheres into predetermined patterns. In this approach, micro-spheres dispersed in a solvent are allowed to sediment onto the pre-patterned substrate and organize in the holes to create a colloidal crystal film that replicates the pattern of the polymer resist. The method is time consuming and the patterned colloidal crystal film grows on top of the substrate. Order of micro-spheres in the colloidal crystal film appears to be maintained only close to the patterned substrate. There is no control over layer thickness by this technique. Moreover sophisticated and expensive photolithography techniques are required to pre-pattern a periodic array of micron scale holes in the substrate to perform colloidal epitaxy.
Another method uses an elastomeric mold containing micron scale channels placed on a planar substrate into which a solution of micro-spheres is drawn by capillary flow (E. Kim, Y. Xia and G. M. Whitesides, Adv. Mater. 1996, 8, 245). Crystallization of micro-spheres occurs within the channels of the micro-mold to create a patterned colloidal crystal film that replicates that of the micro-mold. The patterned colloidal crystal film grows on top of the substrate and the physical dimensions and degree of order of the patterns are limited by mechanical deformation of the materials comprising the channel walls of the micro-mold and the adherence of microspheres to the micro-mold.
Patterned self-assembled monolayers of alkanethiolates on gold substrates (J. Aizenberg, P. V. Braun and P. Wiltzius, Phys. Rev. Lett. 2000, 84, 2997) and polyelectrolytes on gold supported patterned self-assembled monolayers (K. M. Chen, X. Jiang, L. C. Kimerling and P. T. Hammond, Langmuir 2000, 16, 7825) have been used to generate colloidal crystal arrays. By placing functional groups at the end of the alkanethiolate or on the polymer electrolyte that bear positive and/or negative charges it is possible to organize oppositely charged micro-spheres into patterns that replicate those of the alkanethiolate or polyelectrolyte. The micro-spheres however are not ordered into a colloidal crystal by these techniques and there is no control over layer thickness or how one layer is organized with respect to another. In another approach, microspheres have been electroosmotically pumped into a micro-fluidic system of channels fabricated on a glass substrate (R. D. Oleschuk, L. L. Shultz-Lockyear, Y. Ning and D. J. Harrison, Anal. Chem. 2000, 72, 585). This creates an on-chip solid phase for capillary electrochromatography applications. There is no evidence that the micro-spheres are ordered into a patterned colloidal crystal using this approach.
In another strategy a two-dimensional lattice of micro-spheres have been arranged on a planar substrate into a regular pattern one-sphere-at-a-time by micromanipulation in a scanning electron microscope (H. T. Miyazaki, H. Miyazaki, K. Ohtaka, and T. Sato, J. AppI. Phys. 2000, 87, 7152). This method of robotic assembly can sort micro-spheres by size and selectively place them into pre-determined 2-D patterns, however the method is serial in nature, time consuming, expensive and limited so far to a single layer and a single shape of limited size on top of the substrate.
Thus, while there are current existing methods directed to teaching how to pattern micro-spheres and colloidal crystals on substrates, each of these methods has limitations, which are considered to render them unsuitable for the large scale manufacture of photonic-based devices and optically integrated photonic crystal circuits and chips.
Therefore it would be very advantageous to provide methods to produce a patterned and well-ordered colloidal crystal film with controlled thickness, lattice structure, area, orientation and registry, and upon or embedded within wafers of different types, and use such colloidal crystals for templating replica patterns of inverse colloidal crystals, making structural defects of different type in photonic crystals, and positioning and integrating photonic crystal devices with optical waveguides and fibers upon or within wafers. Moreover, the method should be simple, fast, reproducible and inexpensive to implement, easily integrated into existing chip fabrication facilities and readily adaptable for manufacturing photonic chips that couple to optical waveguides and fiber systems. This is not true of any of the methods described in the prior art.